Zea mays: Discovery of Transposons
Zea Mays as a Model Organism A model organism is a non-human organism used to study a particular aspect of biology. An example of a model organism is Zea Mays. Zea Mays (maize) use as model organism can be traced back to experiments done in 1869 by Gregor Mendel which were done to verify his breeding experiments w ith pea plants (1). 30 years later experiments, done with maize, which were conducted by Carl Correns and Hugo de Vries lead to the rediscovery of Mendel's laws of heredity (2). However it was R.A. Emerson and E.M. East who established maize as a model organism (2). Maize is an especially good model for genetic analysis for several reasons. Maize develops with both female and male flowers on separate stems (Figure 1). This separation of male and female flowers allows for great control of pollination and thus breeding. A single pollination can yield hundreds of seeds. Other species of plants are more difficult to work with and yield far fewer seeds per pollination (2). Another benefit maize has is that meiosis is synchronized, so that the size of the developing anthers is correlated to the meiotic stage of the developing pollen. This coupled with the large size of the chromosomes and their highly repetitive heterochromatic regions, which stain darkly, have allowed for meiotic mutants to be identified as well as the chromosomal locations of genes to be determined (2). A final advantage is that maize kernel morphology and composition are quantitative traits which have been used in hundreds of genetic studies (2). Kernels have been used to study transposable elements in DNA (3) as well as biosynthetic pathways (4). More recent developments include the identfication and organization of mutants into a library, the development of a recombinant inbred line (IBM) (5), and the sequencing of the recombinant genome (6). These developments have greatly enhanced maize as a model organism. Transposons and Their Discovery Transposons or transposable elements are segments of DNA that can move within the genome (7). Transposons were first discovered by Barbra McClintock in the 1940s through the study of maize kernels. The stu dy examined the color of the kernels (Figure 2). Kernel color is dependent upon three alleles due to the formation of an endosperm which is triploid. The endosperm gives rise to the outer protein layer of the kernel which is responsible for the color of the kernel (8). Previous research had shown that maize had genes that would encode for multicolored kernels but the reason for this was unknown . In order to investigate this McClintock looked at genes controlling for the color of the kernels (Figure 3) which had been mapped to the short arm of chromosome 9. In order to do this McClintock crossed female maize that were homozygous recessive for kernel color but missing the alleles for a chromosome 9 breakage point with males that were homozygous dominant for lack of color which also had the allele for chromosome 9 breakage (Figure 4). It was expected that the heterozygous offspring would have colorless kernels but some of the kernels had dark brown spots or streaks. McClintock believed that this was due to chromosomal breakage that lead to the loss of the allele preventing color expression and the allele coding for purple color development (8). The degree of color depended on when the breakage occurred. If breakage occurred early in development then a greater degree of color would be seen in the kernel where as if breakage occurred later in development the kernel would have less color (8). McClintock was also able to map the chromosomal breakage point, Ds, and found that its ability to cause chromosomal breakage was dependent upon another gene, Ac. McClintock wa s able to map Ds and Ac and found that their position within the chromosome changed from plant to plant. Additional experiments revealed that Ds actually moved within the chromosome and was able to inhibit the expression of genes, such as Bz, by being inserted into the allele. Furthermore when Ds was removed from the allele, the allele could successfully be expressed (8). However, Ds could only move when Ac was present. The degree of color seen in the kernels was dependent upon when Ds was inserted or excised. This experiment was the first to demonstrate the existence of transposable DNA elements and how they could influence gene expression (8). A Better Understanding of the Ac/Ds System and Transposons The Ac/Ds system can be classified as a type II transposon. Type II transposons, also known as DNA transposons, are genes that encode for an enzyme known as a transposase. This enzyme will cut out the transposon which is then ligated into a new site (7). In the case of the Ac/Ds system it was discovered that Ds is a trunked version of Ac and did not code for a functioning transposase. When Ac was present the functional transposase it codes for was able to excise Ds and allow it to move in the genome (7). Additionally there are tranposons that use an RNA intermediate. These transposons, classified as type I transposons, also known as retrotransposons, use an RNA intermediate that is then reverse transcripted by a reverse transcriptase, which is usually encoded by the transposon and then inserted into the genome (7). There are two types of class I transposons, LTR and non-LTR transposons. LTR transposons have long terminal repeats on both ends while non-LTR transposons do not (8). Non-LTR transposons are the only active type of transposons in humans (9). Sources 1. Coe EH. The origins of maize genetics. Nat Rev Genet 2001; 2: 898-905. 2. Strable J, Scanlon MJ. Maize (Zea Mays): A model organism for basic and applied research in planet biology. Cold Spring Harb Protoc 2009; 4(10): 1-9. 3. McClintock B. The origin and behavior of mutable loci in maize. Proc Natl Acad Sci 1950; 36: 344-355. 4.Scanlon MJ, Stinard PS, James MG, Myers AM, Robertson DS. Genetic analysis of 63 mutations affecting maize kernel development isolated from mutator stocks. Genetics 1994; 136: 281-294. 5.Lee M, Sharopova N, Beavis WD, Grant D, Katt M, Blair D, Hallauer A. Expanding the genetic map of maize with the intermated B73 X Mol17 (IBM) population. Plant Mol Biol 2002; 48: 453-461. 6. Schnable PS et. al. The B73 maize genome: complexity, diversity, and dynamics. Science 2009; 326(5956): 1112-1115. 7. Lodish, H. Berk, A., Kaiser, C.A., Krieger, M., Scott, M.P., Bretscher, A., Ploegh H., Matsudaira, P. Molecular Cell Biology. 6th ed. New York: W.H. Freeman and Company, 2008. Print. 8. Pray L, Zhaurova K. Barbara McClintock and the Discovery of Jumping Genes (Transposons). Nature Education; 1(1). 9. Pray L. Transposons: The jumping genes. Nature Education 2008; 1(1).